This invention relates to the field of binary data transmission and has particular application to multiprocessing systems wherein strings of binary data must be transmitted at high speed between different stations in the network. A number of different data encoding techniques are known in the prior art. Some of the better known codes are the NRZ Code wherein the data signal is high during the entire "mark" interval and is low during the entire "space" interval and the NRZI Code which has a transition for each "space" interval and no transition for "mark" intervals. Another prior art code is the RZ Code which returns to zero during each marking interval. Yet another self-clocking code is the Manchester Code wherein there is a positive transition for each "space" and a negative transition for each "mark". In a variation of the Manchester Code known as a Bi-Phase Mark code, each bit period begins with a transition. There is also a signal transition in the middle of each "mark" bit period. Other well-known codes are Bi-Phase Space, Miller and MFM codes. Each of these codes has associated disadvantages. The RZ, Bi-Phase Mark, Bi-Phase Space, Manchester and MFM codes all require a channel bandwidth which is twice the basic data frequency. This is a serious disadvantage in high speed communication. The NRZ, RZ and NRZI codes are not self-clocking, and are therefore difficult to handle at the receiving end. The NRZ, RZ, NRZI, and MFM codes are not AC coupled. In other words, the data signal may assume a given state for a period of time sufficient to require DC processing. The Miller Code has a problem in that self-clocking may be achieved only with the assistance of a phase locked loop. None of the above-noted codes provides an intermessage space which is unique from the message and which may be detected as such.
It is therefore seen that there is a need for an improved apparatus and method for data transmission which is self-clocking, AC coupled, able to provide a unique intermessage space, and operable at a bandwidth no greater than the data frequency.